conflicts: add "snapshot" conflict marker style

Adds a new "snapshot" conflict marker style which returns a series of
snapshots, similar to Git's "diff3" conflict style. The "snapshot"
option uses a subset of the conflict hunk headers as the "diff" option
(it just doesn't use "%%%%%%%"), meaning that the two options are
trivially compatible with each other (i.e. a file materialized with
"snapshot" can be parsed with "diff" and vice versa).

Example of "snapshot" conflict markers:

```
<<<<<<< Conflict 1 of 1
+++++++ Contents of side jj-vcs#1
fn example(word: String) {
    println!("word is {word}");
------- Contents of base
fn example(w: String) {
    println!("word is {w}");
+++++++ Contents of side jj-vcs#2
fn example(w: &str) {
    println!("word is {w}");
>>>>>>> Conflict 1 of 1 ends
}
```

CHANGELOG.md

@@ -66,6 +66,8 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 * New `ui.conflict-marker-style` config option to change how conflicts are
   materialized in the working copy. The default option ("diff") renders
   conflicts as a snapshot with a list of diffs to apply to the snapshot.
+  The new "snapshot" option just renders conflicts as a series of snapshots,
+  showing each side of the conflict and the base(s).
 
 ### Fixed bugs
 

cli/src/config-schema.json

@@ -163,7 +163,8 @@
                     "type": "string",
                     "description": "Conflict marker style to use when materializing conflicts in the working copy",
                     "enum": [
-                        "diff"
+                        "diff",
+                        "snapshot"
                     ],
                     "default": "diff"
                 }

lib/src/conflicts.rs

@@ -238,6 +238,8 @@ async fn materialize_tree_value_no_access_denied(
 pub enum ConflictMarkerStyle {
     /// Style which shows a snapshot and a series of diffs to apply.
     Diff,
+    /// Style which shows a snapshot for each base and side.
+    Snapshot,
 }
 
 pub fn materialize_merge_result<T: AsRef<[u8]>>(
@@ -272,7 +274,7 @@ pub fn materialize_merge_result_to_bytes<T: AsRef<[u8]>>(
 
 fn materialize_conflict_hunks(
     hunks: &[Merge<BString>],
-    _conflict_marker_style: ConflictMarkerStyle,
+    conflict_marker_style: ConflictMarkerStyle,
     output: &mut dyn Write,
 ) -> io::Result<()> {
     let num_conflicts = hunks
@@ -304,6 +306,15 @@ fn materialize_conflict_hunks(
                     write_base(&base_str, left, output)?;
                     continue;
                 };
+
+                // For any style other than "diff", always emit sides and bases separately
+                if conflict_marker_style != ConflictMarkerStyle::Diff {
+                    write_side(add_index, right1, output)?;
+                    write_base(&base_str, left, output)?;
+                    add_index += 1;
+                    continue;
+                }
+
                 let diff1 = Diff::by_line([&left, &right1]).hunks().collect_vec();
                 // Check if the diff against the next positive term is better. Since
                 // we want to preserve the order of the terms, we don't match against